Nonablative hematopoietic stem cell transplantation (HCT) with an unrelated donor graft has greatly expanded the number of patients suffering from hematologic diseases and malignancies who can be cured by HCT. However, current dosing methods for HCT regimens lead to substantial interpatient variability in the systemic exposure and commensurate percent donor T-cell chimerism. The parent grants have compelling data that percent donor chimerism is associated with the efficacy and toxicity of nonablative HCT. Our central hypothesis is the pharmacokinetic (PK) and drug-specific pharmacodynamics (PD) of fludarabine and mycophenolate mofetil (MMF), along with relevant metabolites, are biomarkers for donor chimerism. To test this hypothesis, we propose to develop population PK models and limited sampling schedules to characterize an individual patient's systemic exposure and to build popPK-PD models using PD endpoints specific for each drug's activity. We propose three specific aims to be tested in patients receiving fludarabine/total body irradiation conditioning, an unrelated peripheral blood mononuclear cell graft, and postgrafting immunosuppression of MMF and a calcineurin inhibitor. In Aim 1, we will evaluate the PK and PD of plasma fludarabine and intracellular fludarabine triphosphate (FTP), its active metabolite. Using a novel analytical method, we are uniquely poised to measure in-vitro FTP formation by CD4+ and CD8+ cells obtained from patients awaiting HCT and evaluate the cellular mechanisms of FTP formation. We will evaluate fludarabine- specific PD, specifically evaluating the relationship of these PK endpoints with the decline in circulating CD4+ and CD8+ cells. In Aim 2, we will characterize the PK and PD of MMF's active metabolite, mycophenolic acid (MPA), and to confirm our findings that a low MPA AUC increases the risk of low percent donor chimerism and rejection. We also propose to evaluate the genetic mechanisms of MPA clearance and to characterize the PD of MPA and inosine monophosphate dehydrogenase (IMPDH) activity, the enzyme inhibited by MPA. In Aim 3, we propose to evaluate if percent donor T-cell chimerism is associated with the PK and drug-specific PD biomarkers of fludarabine and MMF. We propose to evaluate the PD of donor T-cell chimerism with the PK and drug-specific PD markers evaluated in Aims 1 and 2. Our individual areas of expertise, in combination with our history of collaborations, demonstrate that we are well-poised to test the central hypothesis, thus providing a unique opportunity for these ancillary studies to improve outcomes for patients with hematologic disease or malignancies receiving HCT. The goal of these studies is to identify patient-specific factors related to the balance between the recipient cells and donor cells in patients who receive a hematopoietic stem cell transplant. We will look at two types of patient-specific factors - how the patients'body breaks down and how their bodies immediately responds - to two drugs, fludarabine and mycophenolate. (End of Abstract)